It is critical for photopolymer composition utilized in making printing plates to exhibit a high degree of water swell which allows for easy scrub-off of that part of the printing plate which was not cured. In other words, it is important for the rubbery polymer employed in making the printing plate to be hydrophilic in nature. It is also highly desirable for the photopolymer compositions utilized in making printing plates to have a high degree of clarity so that ultra-violet light which is transmitted through a photographic negative can penetrate the composition and initiate cure. There are also certain mechanical requirements needed from such photopolymer compositions.
U.S. Pat. No. 4,045,231 discloses the utilization of polystyrene-polybutadiene-polystyrene triblock polymers in printing plates. U.S. Pat. No. 4,162,919 discloses the employment of styrene-isoprene diblock polymers and styrene-isoprene diblock polymers in flexographic printing plates. U.S. Pat. No. 5,344,743 discloses flexographic photosensitive printing plates which are made with formulations comprising triblock polymers of poly(vinylpyridine)-poly(butadiene) or poly(isoprene)-poly(vinylpyridine).
Certain carboxylated nitrile rubbers, such as Chemigum.RTM. NX775 carboxylated nitrile rubber which is sold by The Goodyear Tire & Rubber Company, offer most of the physical requirements needed from a base polymer for printing plate photopolymer compositions. However, such carboxylated nitrile rubbers do not have adequate clarity and are too water-resistant for utilization in such applications. It is also important for carboxylated nitrile rubbers utilized in such applications to have a Mooney ML-4 viscosity of less than about 80.
Except for their inadequate clarity and water swell characteristics, the carboxylated nitrile rubbers disclosed by U.S. Pat. Nos. 4,415,690, 4,435,535 and 4,452,936 would be suitable for utilization in making printing plate photopolymer compositions. U.S. Pat. No. 4,415,690 discloses the utilization of succinic anhydride derivatives as scorch inhibiting agents. U.S. Pat. No. 4,435,535 discloses the utilization of adipic acid as a scorch inhibiting agent and U.S. Pat. No. 4,452,936 discloses the utilization of oligomerized fatty acids as scorch inhibiting agents.
United States patent application Ser. No. 08/766,449 pending, discloses a carboxylated nitrile rubber which is suitable for utilization as a base polymer for printing plate photopolymer compositions. The level of methacrylic acid incorporated into this rubber is increased so as to be within the range of about 10 weight percent to about 20 weight percent to increase water swell. Ser. No. 08/766,449 pending also reports that it is critical for the coagulation process carried out in recovering the carboxylated nitrile rubber from the latex to be accomplished without utilizing polyelectrolytes, such as Nalco.TM. 108 polyelectrolyte, as coagulation aids. The utilization of at least one organic acid which contains at least three carboxyl groups, such as citric acid, in the carboxylated nitrile rubbers is also reported as being critical.
United States patent application Ser. No. 08/766,449 pending more specifically discloses a high clarity water-sensitive carboxylated nitrile rubber composition which is comprised of (a) a carboxylated nitrile rubber which is comprised of repeat units which are derived from about 30 weight percent to about 60 weight percent 1,3-butadiene, from about 30 weight percent to about 50 weight percent acrylonitrile and from about 10 weight percent to about 20 weight percent methacrylic acid and (b) from about 1 to about 10 phr of at least one organic acid which contains at least three carboxyl groups. The carboxylated nitrile rubber of Ser. No. 08/766,449 pending is reported to typically have a Mooney ML-4 viscosity of less than about 80.